Embodiments of the invention relate generally to heat treatment of semiconductor wafers and other substrates. In particular, embodiments of the invention relate to rapid thermal processing of wafers from a radiant source, such as an array of incandescent lamps.
The fabrication of integrated circuits from silicon or other wafers involves many steps of depositing layers, photo lithographically patterning the layers, and etching the patterned layers. Ion implantation is used to dope active regions in the semiconductive silicon. The fabrication sequence also includes thermal annealing of the wafers for many uses including curing implant damage and activating the dopants, crystallization, thermal oxidation and nitridation, silicidation, chemical vapor deposition, vapor phase doping, thermal cleaning, and other reasons. Although annealing in early stages of silicon technology typically involved heating multiple wafers for long periods in an annealing oven, rapid thermal processing, (RTP) has been increasingly used to satisfy the ever more stringent requirements for ever smaller circuit features. RTP is typically performed in single-wafer chambers by irradiating a wafer with light from an array of high-intensity lamps directed at the front face of the wafer on which the integrated circuits are being formed. The radiation is at least partially absorbed by the wafer and quickly heats it to a desired high temperature, for example above 600° C., or in some applications, above 1000° C. The radiant heating can be quickly turned on and off to controllably and uniformly heat the wafer over a relatively short period, for example, of a minute or less, or even a few seconds. RTP chambers are capable of uniformly heating a wafer at rates of about 50° C./second and higher, for example, at rates of 100°-150° C./second, and 200°-400° C./second. Typical ramp-down (cooling) rates in RTP chambers are in the range of 80-150° C./second. Some processes performed in RTP chambers require variations in temperature across the substrate of less than a few degrees Celsius.
Since rapid thermal processing works on a single semiconductor each time, optimal heating and cooling means are necessary for optimal RTP performance. It is desirable to optimize substrate temperature uniformity during thermal processing of the substrate. Temperature uniformity provides uniform process variables on the substrate (e.g. layer thickness, resistivity, etch depth) for temperature activated steps such as film deposition, oxide growth and etching. In addition, substrate temperature uniformity is necessary to prevent thermal stress-induced substrate damage such as warpage, defect generation and slip. For example, at 1150° C., a center to edge temperature difference on a four-inch silicon wafer of approximately 5° C. can induce dislocation formation and slip. Temperature gradients may also be induced by other sources. For example, a substrate may have non-uniform emissivity because of spatial modifications to surface areas or volumes of the substrate. These modifications may include films that have been patterned by photolithography or locally doped regions, such as buried layers for bipolar transistors. In addition, substrate temperature gradients may be induced by localized gas cooling or heating effects related to processing chamber design as well as non-uniform endothermic or exothermic reactions that may occur on the substrate surface during processing. It would be desirable to provide RTP chambers that provide improved temperature uniformity.